This invention relates in general to imaging systems and more particularly to improved electrostatographic developer mixtures for use in such systems.
The formation and development of images on the surface of photoconductive materials by electrostatographic means is known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light, and developing the resultant electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner is attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This "powder" image may then be transferred, usually electrostatically, to a support surface such as paper. The transferred image may subsequently be permanently affixed to a support surface by heat or other suitable affixing means, such as solvent or overcoating treatment may be used instead.
Many methods are known for applying the electroscopic particles to the latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,582, is known as "cascade" development. In this method, developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the surface bearing the electrostatic latent image. The charged portions of the surface have a charge of the same polarity as, but stronger, than, the carrier particles. Toner and carrier particles having opposite polarities are selected so that the toner particles cling to the carrier particles. In order to develop a negatively charged electrostatic latent image, a toner and carrier combination are selected in which the toner is triboelectrically positive in relation to the carrier. Conversely, to develop a positively charged electrostatic latent image, a toner and carrier combination wherein the toner is triboelectrically negative in relation to the carrier is used. The triboelectric relationship between the toner and carrier depends on the relative positions of the materials in the "triboelectric series". In this series, materials are arranged in ascending order of their ability to take on a positive charge. Each material is positive with respect to any material classified below it in the series; and, negative with respect to any material above it in the series. As the developer mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically attracted from the carrier to the charged portions of the image-bearing surface, whereas they are not electrostatically attracted to the uncharged or background portions of the image which they contact. The carrier particles and unused toner particles are then recycled. The cascade development process is extremely good for the development of line copy images, and is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed for example, in U.S. Pat. No. 2,874,063. In this process, a developer material containing toner and magnetic carrier particles is attracted to and is carried by a magnet. The magnetic field causes alignment of the magnetic particles in a brush-like configuration. When this magnetic brush is brought into contact with an electrostatic latent image-bearing surface, the toner particles are attracted from the carrier particles of the brush to the charged areas of the image-bearing surface but not to the uncharged areas. Since the charged areas have an imagewise configuration, the toner material clings to the surface in imagewise configuration, thus developing the latent image.
Anoter method for developing electrostatic latent images is disclosed in U.S. Pat. No. 3,503,776 issued to R. W. Gundlach. In this method, images are formed by transporting an electrostatic latent image-bearing surface in a generally ascending arcuate path, and contacting only the image in a contact zone with a bath of developer material transported in a concave chamber adjacent the lower path of the imaging surface. The contact zone extends from about the lowermost point of the arcuate path to the uppermost point of the arcuate path. As the imaging surface is transported along its arcuate path, frictional contact between the developer and the imaging surface in the contact zone circulates the developer in the bath and brings developer material into developing configuration with the imaged surface.
Many other methods, such as the "touchdown" development method disclosed by C. R. Mayo in U.S. Pat. No. 2,895,847 are known for applying electroscopic particles to the electrostatic latent image to be developed. The development process, as described above, together with numerous modifications, are well-known to the art through various patents and publications and through the widespread availability and utilization of electrostatographic imaging equipment.
In automatic reproduction equipment, it is conventional to employ as the imaging plate a photoconductor on a conductive substrate in the form of a cylindrical drum of a flexible belt which is continuously rotated through a cycle of sequential operations including charging, exposing, developing, transferring and cleaning. The developer chamber is charged with a developer mixture comprising carrier particles and enough toner particles for hundreds of reproduction cycles. Generally, the freshly charged developer mixtures contain between about 1.5 and 5% toner particles based upon the weight of the developer. This initial concentration provides sufficient toner for many reproduction cycles without causing undesirably high background toner deposition.
While ordinarily capable of producing good quality images, conventional developing systems suffer serious deficiencies in certain areas. In the reproduction of high contrast copies such as letters, tracings and the like, it is desirable to select the electroscopic powder and carrier materials so that their mutual electrification is relatively large; the degree of such electrification being governed in most cases by the distance between their relative positions in the triboelectric series. However, when otherwise compatible electroscopic powder and carrier materials are removed from each other in the triboelectric series by too great a distance, the resulting images are very faint because the attractive forces between the carrier and toner particles compete with the attractive forces between the electrostatic latent image and the carrier particles. Although the image density described in the immediately preceding sentence may be improved by increasing toner concentration in the developer mixture, undesirably high background toner deposition as well as increased toner impaction and agglomeration is encountered when the developer mixture is over-toned. The initial electrostatographic plate charge may be increased to improve the density of the deposited powder image, but the plate charge would ordinarily have to be excessively high in order to attract the electroscopic powder away from the carrier particles. Excessively high electrostatographic plate charges are not only undesirable because of the high power consumption necessary to maintain the electrostatographic plate at high potentials, but also because the high potential causes the carrier particles to adhere to the electrostatographic plate surface rather than merely roll across and off the electrostatographic plate surface. Print deletion and massive carry-over of carrier particles often occur when carrier particles adhere to reusable electrostatographic imaging surfaces. Massive carrier carry-over problems are particularly acute when the developer is employed in solid area coverage machines where excessive quantities of toner particles are removed from carrier particles thereby leaving many carrier particles substantially bare of toner particles. Further, adherence of carrier particles to reusable electrostatographic imaging surfaces promotes the formation of undesirable scratches on the surfaces during image transfer and surface cleaning operations. It is, therefore, apparent that many materials which otherwise have suitable properties for employment as carrier particles are unsuitable because they possess too high a triboelectric value. In addition, uniform triboelectric surface characteristics of many carrier surfaces are difficult to achieve with mass production techniques. Quality images are, in some instances almost impossible to obtain in high speed automatic machines when carriers having non-uniform triboelectric properties are employed. Although it may be possible to alter the triboelectric value of an insulating carrier material by blending the carrier material with another insulating material having a triboelectric value remote from the triboelectric value of the original carrier material, relatively larger quantities of additional material are necessary to alter the triboelectric value of the original carrier material. The addition of large quantities of material to the original carrier material to change the triboelectric properties thereof requires a major manufacturing operation and often undesirably alters the original physical characteristics of the carrier material. Further, it is highly desirable to control the triboelectric properties of carrier surfaces to accommodate the use of desirable toner compositions while retaining the other desirable physical characteristics of the carrier. The alteration of the triboelectric properties of a carrier by applying a surface coating thereon is a particularly desirable technique. With this technique, not only is it possible to control the triboelectric properties of a carrier made from materials having desirable physical characteristics, it is also possible to employ materials previously not suitable as a carrier. Thus, for example, a carrier having desirable physical properties with the exception of hardness, can be coated with a material having desirable hardness as well as other physical properties, rendering the resultant product more useful as a carrier. However, since most carrier coating materials are deficient in one or more of the above areas, there is a continuing need for improved electrostatographic carrier and developer compositions and methods for forming the same.